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From crustal thickening to orogen-parallel escape: the 120 Ma-long HT-LP evolution of the Paleozoic Famatinian back-arc, NW Argentina
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  • Pablo Guillermo Farias,
  • Roberto F. Weinberg,
  • Alfonso Sola,
  • Raúl Becchio
Pablo Guillermo Farias
Monash University, Monash University

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Roberto F. Weinberg
Monash University, Monash University
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Alfonso Sola
Universidad de Salta. IBIGEO-CONICET, Universidad de Salta. IBIGEO-CONICET
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Raúl Becchio
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Exposed sections of accretionary orogens allow reconstruction of their tectonic evolution. Most commonly, orogens are characterised by two-dimensional shortening perpendicular to the orogenic front. We describe the mid-crustal section of the back-arc of the early Paleozoic Famatinian accretionary orogen, exposed in the Sierra de Quilmes. Here crustal deformation evolved from a typical two-dimensional shortening with tectonic transport towards the west, to a non-coaxial constrictional strain with a southward tectonic transport parallel to the orogen. During the early phase of deformation, HT-LP metamorphic complexes were juxtaposed by west-directed thrusting on remarkably thick shear zones forming a thrust duplex. Deformation of the buried footwall complex continued after the exhumed hanging wall ceased to deform. We suggest that the thermally-weakened footwall complex responded by initiating a phase of south-verging thrusting, parallel to the orogen, associated with strong constriction, associated with L-tectonites, and sheath folds. This late phase of deformation defines a non-coaxial constrictional regime characterized by simultaneous east-west and vertical shortening and strong north-south, orogen-parallel stretching. Titanite ages and Zr-in-titanite thermometry demonstrate that this back-arc remained above 700 °C for 120 Ma between 500 and 380 Ma. Combined with regional geology, the new data suggest that west-verging thrusting interrupted an early, back-arc extensional phase, and lasted from ~ 470 to 440 Ma, and that footwall constriction and south-verging thrusting continued for another 40 to 60 Ma. The Famatinian back-arc exposed in Sierra de Quilmes thus is an example of how shortening and orogenic growth in a hot orogen was counterbalanced by lateral flow.